Process for heat treating ferrous metals



Oct. 13,1936. w N M YHEW 2,051,274

PROCESS FOR HEAT TREATING FERROUS METALS Filed May 16, 1953 awn m nun 55$: %ZZQC M15077 Ma I561).

' a) I 7 V Patented Oct. 13, 1936 UNITED STATES PATENT OFFICE PROCESS FOR HEAT TREATING FERROUS METALS Claims.

The principal object of the present invention is to provide for controllably changing the physical properties hardness, tensile strength and ductility-of the metal, and thereby, in some 5 cases, as with structural steel, obtaining greater duty with the same weight or the same duty with less weight, and in other cases increasing the hardness and structural strength as in rails, gears, tools and the like.

' Other objects of the present invention will appear from the following description at the end of which the invention will be claimed.

Generally stated the invention consists in treating ferrous metal by contacting it while hot with a water solution containing nitrogen compounds, and liberating both nitrogen and hydrogen of combination in the water from the solution at the surface of the metal, the hydrogen operating to deeply penetrate the metal with the admixture of hydrogen and nitrogen imparting hardness and brittleness to the metal, and then applying oxygen at the surface of the metal which by combination with the occluded hydrogen extracts it and tempers the brittleness and restores ductility to the metal.

The invention also comprises the improvements to be presently described and finally claimed.

To comply with the rules of the Patent Oflice,

a; drawing accompanies this specification, and in Figure 1 is a diagrammatic or schematic view principally in section and illustrative of apparatus for a galvanic action of a local character.

Fig. 2 is a similar view illustrative of apparatus for a short circuited galvanic action, and

Fig. 3 is a similar view illustrative of a galvanic action in connection with applied current.

Examples of ferrous metals treated by the present invention to change their hardness and tensile strength, ductility and other physical properties are ferrous metals and ferrous alloys which can withstand sudden changes of temperature encountered in the treatment.

The invention is practiced, (l) by immersion of the heated metal at temperatures varying from 1000 degrees Fahrenheit to 2200 degrees Fahrenheit more or less in, or by passing it through a stream or a jet, or streams or jets, of an 5e aqueous solution containing nitrogen, such as cyanicles, cyanates, nitrates, nitrites of sodium, potassium, calcium and the like and ammoniacai compounds, amines and the like. These variant temperatures are one method of control of the changes in hardness, tensile strength, ductility and physical properties. The temperature of the solution used varies from 40 degrees Fahrenheit to 100 degrees Fahrenheit more or less, this variation in temperature being another method of control, and the period of immersion or its 5 equivalent in passing through the streams or jets is another method of control.

The purpose of this first step is to utilize the hydrogen released from the aqueous solution as a carrier for deeply nitriding the metal, whereby 10 it is made hard and brittle, but the brittleness is reduced with consequent restoration of duetility by the second step to be presently described.

The invention is also practiced and completed, (2) by immersion into, or passage through 15 streams or jets of aqueous solutions containing chemically combined oxygen, such as potassium permanganate (KMnOr) sodium nitrate (NaNOa), and chromic acid (HZCIOi), using the controls given in the preceding paragraph. The oxygen liberated combines with the hydrogen 29 occluded in the metal and reduces the brittleness leaving the nitrogen inclusion in the metal.

By immersing the heated metal into any of the above or similar compounds or electrolytes, use is made of the electro-motive force generated to 25 release nitrogen, hydrogen, or oxygen or other, gas, either by localized or internal currents, or produced by short circuiting the metal to be treated with the container of the electrolyte, thus providing a method of control for the physical 30 properties, hardness, tensile strength and ductility of the metal.

I may superimpose on the heated metal being so treated, an external, direct electro-motive force, of varying value of current density or fre- 35 quency. With the specimen so treated with direct current, as outlined above, I found that the reversal of polarity and the varying voltage and current density were other methods of con trol of the hardness, tensile strength, ductility and physical properties. By making the metal the anode hydrogen is disassociated at its surface and induces deep nitriding and by making the metal the cathode oxygen is disassociated at its surface and by combination removes hydrogen. 45 In this paragraph reference is made to the two step process consisting of deeply nitriding in the presence of hydrogen and removing occluded hydrogen by the application of oxygen.

By means of the above processes I have increased the Rockwell hardness of 0.25% carbon steel from 60 to 67 on the B scale, for an untreated specimen, even up to from 45 to 52 on the 0 scale for 2. treated specimen.

I have also by the means outlined above increased the ultimate tensile strength of untreated metal, averaging from 59,000 to 60,000 pounds per square inch even up to 178.500 to 187,800 pounds per square inch with proportionate increase in the elastic limit. These ultimate tensile strength tests were made on a Riehle universal screwtesting machine.

By way of further description and not limitation, one example of an increase in hardness and physical properties was as follows: A standard .505 inch tensile specimen (carbon 0.26) heated to 1800 degrees Fahrenheit was immersed for five seconds in a solution of sodium cyanide (NaCNl of a strength of one pound of cyanide to one gallon of water, and immediately removed from this solution and immersed into a concentrated solution of chromic acid H. .CrO and allowed to cool therein. (The temperature of bothsolutions was about fifty degrees Fahrenheit.) The untreated specimen had an average hardness on the Rockwell B scale of and this was increased to 46 on the C scale for the treated specimen. The tensile strength was increased from 59,500 pounds per square inch to 187,800 pounds per square inch.

Upon examination of the micro-structure of the cross section of the treated specimen I found a gradient of dark areas from the center to the outside of the specimen difiering radically from a case, or case hardening, the darkest areas were at the outside and indicated a nitrogen and/or other occlusion or inclusion. This is borne out by Rockwell hardness tests which showed a gradient from a hard exterior to a softer interior having no sudden transition as is found in a case.

By varying one or other of the various controls, as outlined above, and only increasing the ultimate tensile strength to about 90,000 pounds per square inch, I have increased the ductility of the treated specimen about 10% greater than that of the untreated specimen. In this para graph reference is made to the two step process hereinabove defined and the controls effect the deepness of the penetration in the nitriding step and the removal of occluded hydrogen in the second step. The deeper the nitriding with hydrogen the greater the increase in tensile strength accompanied with hardness and brittleness and the greater the removal of hydrogen the greater the consequent recovery in ductility.

By methods above described I have hardened what is known as Armco iron (being a practically pure iron, as rolled by the American Rolling Mill Company) from 50 for the untreated specimen on the Rockwell 13" scale to for the treated specimen on the same scale, and have increased the ultimate tensile strength from 42,000 pounds per square inch to 58,000 to '78,500 pounds per square inch by one or the other of the processes as outlined above.

In Fig. l the E. M. F. is locally generated between the electrolyte I and metal 2. In Fig. 2 the metal 2. is short circuited with the container 3, and in Fig. 3 current is applied to the metal 2 and to the container 3 as indicated at 4.

Concentrated solutions or electrolytes may well be used, but the degree of concentration is another means of control of the physical properties of the treated metal.

It will be obvious to those skilled in the art to which the invention relates that modifications may be made in details of construction and arrangement and matters of mere form without departing from the spirit of the invention which is not limited to such matters, or otherwise than the prior art and the appended claims may require.

I claim:

1. The improvement in the process of treatin ferrous metals which consists in contacting the metal while hot with a water solution containing a nitrogen compound and deeply penetrating the metal by liberating in contact therewith nitrogen and hydrogen which carries the nitrogen deeply into the metal making the same brittle and hard, and extracting hydrogen leaving the nitrogen by liberating oxygen from a solution containing it in contact with the treated metal.

2. The process which consists of deeply impregnating ferrous metal with nitrogen which consists in simultaneously liberating nitrogen from an aqueous solution containing nitrogen compounds and hydrogen by electrolysis of the water of the solution in contact with the ferrous metal while hot, and extracting hydrogen from the metal by liberating oxygen from a solution containing it in contact with the treated metal.

3. In the process described in claim 2 the extraction of the hydrogen from the treated metal by making it the anode in the electrolysis thereby releasing oxygen from the aqueous solution which extracts hydrogen from the metal.

4. The process which consists in nitriding ferrous metal by the electrolysis of a water solution containing nitrogen compound thereby liberating nitrogen from the compound and hydrogen from the water in contact with the hot metal by an applied E. M. F. and subjecting the surface of the treated metal to contact with oxygen which extracts hydrogen from the treated metal by chemical combination with the hydrogen.

5. The improvement in the process of treating ferrous metals which consists in contacting the metal while at a temperature above one thousand degrees Fahrenheit with a solution containing a nitrogen compound and deeply penetrating the metal by liberating in contact therewith nitrogen and hydrogen which carries the nitrogen deeply into the metal making the same brittle and hard and extracting hydrogen by contacting oxygen with the surface of the treated metal while hot.

WALLACE NELSON MAYHEW.- 

